Amplifiers use power transistor devices to develop an amplified version of an input signal. The input signal is often modulated to carry information. To avoid distortion or other loss of the information, the power transistor device is configured and operated to amplify the input signal in a linear manner.
Power transistor devices are often controlled to improve operational characteristics. For instance, a power transistor device may be controlled to limit the input signal or bias voltages to levels at which the power transistor device operates in an efficient and linear manner. In linearization control schemes, the output power of the power transistor device is often monitored. Output monitoring is used to ensure that the modulation component of the input signal in the amplified output signal is acceptably linear.
Radio frequency (RF) power couplers are used for power monitoring. Some RF power couplers are installed to monitor the forward power delivered to the load. The forward power level measurement may then be used in a feedback loop to control the power transistor device.
RF power couplers are also installed to monitor the reflected power from the load. Such monitoring may be used to protect the power transistor devices from failure during output mismatch. Output mismatch arises with an offset in the output impedance of the amplifier and the input impedance of the load. The amount of output mismatch is identified by monitoring the power reflected from an output load.
RF power couplers often use distributed transmission lines and lumped-element networks. In such cases, the transmission lines may become undesirably long in connection with, for instance, quarter-wavelength transmission lines. For example, a transmission line on the surface of a substrate with a limited dielectric constant can be relatively long for an operating frequency of 2 Gigahertz (GHz). Even with a substrate having a relatively high permittivity, for example, a permittivity equal to about 10, a transmission line length will be of the order of 20 millimeters (mm).
For these reasons, RF power couplers have consumed a significant amount of space, e.g., on a printed circuit board. RF power couplers may also introduce undesirably long time delays in the output signal when being used for power control or linearization.